Ablation of the interior lining of a body organ is a procedure which involves heating the organ lining to temperatures which destroy the cells of the lining or coagulate tissue proteins for hemostasis. Such a procedure may be performed as a treatment to one of many conditions, such as chronic bleeding of the endometrial layer of the uterus or abnormalities of the mucosal layer of the gallbladder. Existing methods for effecting ablation include circulation of heated fluid inside the organ (either directly or inside a balloon), laser treatment of the organ lining, and resistive heating using application of radio-frequency (RF) energy to the tissue to be ablated.
Techniques using RF energy provide an RF electrical signal to one or more electrodes in contact with the subject organ tissue. Electrical current flows from the electrodes and into the organ tissue. The current flow resistively heats the surrounding tissue. Eventually, the heating process destroys the cells surrounding the electrodes and thereby effectuates ablation.
Before the start of power delivery, blood and saline solution may surround the electrodes. As the cells surrounding the electrodes are destroyed, additional blood and saline solution will surround the electrodes. These conductive liquids act to decrease the electrode impedance. These liquids may be suctioned away during the ablation process. Absent these conductive liquids, the electrode impedance will increase with the destruction of the surrounding cells. Depending upon the specific electrode configuration, the impedance characteristics may change from as little as a fraction of an ohm to well over 200 ohms during the course of an ablation procedure.
The RF ablation technique must be performed using great care to prevent over-ablation. Monitoring of tissue surface temperature is normally carried out during these ablation procedures to ensure the temperature does not exceed 100° C. If the temperature exceeds 100° C., the fluid within the tissue begins to boil and to thereby produce steam. Because ablation is carried out within a closed cavity within the body, the steam cannot escape and may instead force itself deeply into the tissue, or it may pass into areas adjacent to the area intended to be ablated, causing embolism or unintended burning of adjacent tissues.
An RF ablation device must accurately determine the appropriate level of power. This power level must provide sufficient heating to effectuate ablation. At the same time, the power level must be controlled to prevent over-ablation. Moreover, an RF generator must be controlled to respond dynamically to changes in the impedance of the subject tissue.
Existing RF ablation devices generally apply power to electrodes having a relatively small surface area (e.g. forcept electrodes). Such electrodes ablate a relatively small surface area at a relatively high impedance. Accordingly, the required RF power is relatively low and simple to deliver (typically 80 Watts at approximately 100 Ohms). During an ablation procedure, the impedance characteristics of such electrodes may change. However, many generators are suitable for providing the relatively low power level over a range of high impedances.
To ablate a large tissue area with electrodes having a relatively small surface area, an operator must move electrodes about the tissue surface. This introduces a measure of imprecision. An electrode matched to the surface area of the subject tissue reduces this imprecision. However, matching the electrode area to the subject tissue area significantly increases the surface area of the electrode. Accordingly, the electrode requires significantly greater power levels to effect ablation. Moreover, relatively large surface areas are characterized by a relatively low initial impedance, which increases significantly during the course of an ablation.
It is therefore desirable to provide a power level sufficient to effectuate ablation on a relatively large surface area electrode. The power level, however, must not result in over-ablation. It is also desirable to dynamically control the RF generator to respond to impedance changes within the subject organ tissue. It is further desirable to provide an ablation method and device which allows the depth of ablation to be controlled reliably and which automatically discontinues ablation once the desired ablation depth has been reached.